The ability of T cells to mediate an immune response against an antigen requires two distinct signaling interactions (Viglietta, V. et al. (2007) Neurotherapeutics 4:666-675; Korman, A. J. et al. (2007) Adv. Immunol. 90:297-339). First, an antigen that has been arrayed on the surface of antigen-presenting cells (APC) is presented to an antigen-specific naive CD4+ T cell. Such presentation delivers a signal via the T cell receptor (TCR) that directs the T cell to initiate an immune response specific to the presented antigen. Second, various co-stimulatory and inhibitory signals mediated through interactions between the APC and distinct T cell surface molecules trigger the activation and proliferation of the T cells and ultimately their inhibition.
The immune system is tightly controlled by a network of costimulatory and co-inhibitory ligands and receptors. These molecules provide the second signal for T cell activation and provide a balanced network of positive and negative signals to maximize immune responses against infection, while limiting immunity to self (Wang, L. et al. (Epub Mar. 7, 2011) J. Exp. Med. 208(3):577-92; Lepenies, B. et al. (2008) Endocrine, Metabolic & Immune Disorders—Drug Targets 8:279-288). Examples of costimulatory signals include the binding between the B7.1 (CD80) and B7.2 (CD86) ligands of the APC and the CD28 and CTLA-4 receptors of the CD4+ T-lymphocyte (Sharpe, A. H. et al. (2002) Nature Rev. Immunol. 2:116-126; Lindley, P. S. et al. (2009) Immunol. Rev. 229:307-321). Binding of B7.1 or B7.2 to CD28 stimulates T cell activation, whereas binding of B7.1 or B7.2 to CTLA-4 inhibits such activation (Dong, C. et al. (2003) Immunolog. Res. 28(1):39-48; Greenwald, R. J. et al. (2005) Ann. Rev. Immunol. 23:515-548). CD28 is constitutively expressed on the surface of T cells (Gross, J., et al. (1992) J. Immunol. 149:380-388), whereas CTLA-4 expression is rapidly up-regulated following T-cell activation (Linsley, P. et al. (1996) Immunity 4:535-543).
Other ligands of the CD28 receptor include a group of related B7 molecules, also known as the “B7 Superfamily” (Coyle, A. J. et al. (2001) Nature Immunol. 2(3):203-209; Sharpe, A. H. et al. (2002) Nature Rev. Immunol. 2:116-126; Collins, M. et al. (2005) Genome Biol. 6:223.1-223.7; Korman, A. J. et al. (2007) Adv. Immunol. 90:297-339). Several members of the B7 Superfamily are known, including B7.1 (CD80), B7.2 (CD86), the inducible co-stimulator ligand (ICOS-L), the programmed death-1 ligand (PD-L1; B7-H1), the programmed death-2 ligand (PD-L2; B7-DC), B7-H3, B7-H4 and B7-H6 (Collins, M. et al. (2005) Genome Biol. 6:223.1-223.7).
The Programmed Death 1 (PD-1) protein is an inhibitory member of the extended CD28/CTLA-4 family of T cell regulators (Okazaki et al. (2002) Curr Opin Immunol 14: 391779-82; Bennett et al. (2003) J. Immunol. 170:711-8). Other members of the CD28 family include CD28, CTLA-4, ICOS and BTLA. Two cell surface glycoprotein ligands for PD-1 have been identified, Program Death Ligand 1 (PD-L1) and Program Death Ligand 2 (PD-L2). PD-L1 and PD-L2 have been shown to downregulate T cell activation and cytokine secretion upon binding to PD-1 (Freeman et al. (2000) J Exp Med 192:1027-34; Latchman et al. (2001) Nat Immunol 2:261-8; Carter et al. (2002) Eur J Immunol 32:634-43; Ohigashi et al. (2005) Clin Cancer Res 11:2947-53).
PD-L1 (also known as cluster of differentiation 274 (CD274) or B7 homolog 1 (B7-H1)) is a 40 kDa type 1 transmembrane protein. PD-L1 binds to its receptor, PD-1, found on activated T cells, B cells, and myeloid cells, to modulate activation or inhibition. Both PD-L1 and PD-L2 are B7 homologs that bind to PD-1, but do not bind to CD28 or CTLA-4 (Blank et al. (2005) Cancer Immunol Immunother. 54:307-14). Binding of PD-L1 with its receptor PD-1 on T cells delivers a signal that inhibits TCR-mediated activation of IL-2 production and T cell proliferation. The mechanism involves inhibition of ZAP70 phosphorylation and its association with CD3ζ (Sheppard et al. (2004) FEBS Lett. 574:37-41). PD-1 signaling attenuates PKC-θ activation loop phosphorylation resulting from TCR signaling, necessary for the activation of transcription factors NF-κB and AP-1, and for production of IL-2. PD-L1 also binds to the costimulatory molecule CD80 (B7-1), but not CD86 (B7-2) (Butte et al. (2008) Mol Immunol. 45:3567-72).
Expression of PD-L1 on the cell surface has been shown to be upregulated through IFN-γ stimulation. PD-L1 expression has been found in many cancers, including human lung, ovarian and colon carcinoma and various myelomas, and is often associated with poor prognosis (Iwai et al. (2002) PNAS 99:12293-7; Ohigashi et al. (2005) Clin Cancer Res 11:2947-53; Okazaki et al. (2007) Intern. Immun. 19:813-24; Thompson et al. (2006) Cancer Res. 66:3381-5). PD-L1 has been suggested to play a role in tumor immunity by increasing apoptosis of antigen-specific T-cell clones (Dong et al. (2002) Nat Med 8:793-800). It has also been suggested that PD-L1 might be involved in intestinal mucosal inflammation and inhibition of PD-L1 suppresses wasting disease associated with colitis (Kanai et al. (2003) J Immunol 171:4156-63).
Given the importance of immune checkpoint pathways in regulating an immune response, the need exists for developing novel agents that modulate the activity of immunoinhibitory proteins, such as PD-L1, thus leading to activation of the immune system. Such agents can be used, e.g., for cancer immunotherapy and treatment of other conditions, such as chronic infection.